(a) Field of the Invention
The present invention relates to a focusing method for a telephoto lens system and, more particularly, to a focusing method for a telephoto lens system with a large aperture ratio.
(b) Description of the Prior Art
In recent years, there is increasing demand for large aperture ratio telephoto lens systems in various fields. A large aperture ratio telephoto lens system enables to take photographs at a high shutter speed. Therefore, when photographing an indoor sport game, stage scene, etc., for example, it is possible to take photographs without causing object shaking and, consequently, the limitation in the photographing conditions is reduced. Besides, as the F-number with the full open aperture is small, it is possible to take a photograph in which the object is relieved by mading the depth of field shallow and putting the things in front and rear of the object out of focus, such photography being characteristic of telephoto lens systems.
However, for a telephoto lens system with a large aperture ratio, the front lens group should be necessarily made large in diameter and should be composed of a large number of lens elements and, consequently, the front lens group becomes heavy. Therefore, in case of the conventional focusing method to advance the lens system as a whole, it is inconvenient for handling in various points, for example, the center of gravity shifts forward at the time of focusing, the helicoid becomes heavy because the amount of advancing movement should be made large as the focal length is long, and so forth.
Besides, the inner focusing method is also known as a focusing method for a telephoto lens system. This method is to focus the lens system by moving a part of lens elements in the rear lens group toward the image side or object side, i.e., by moving one or two lens components. In case of the inner focusing method to move one lens component in the rear lens group, it is difficult to keep the quality of image satisfactorily favourable up to a short distance. On the other hand, in case of the inner focusing method to move two lens components (for example, as disclosed in Japanese published examined patent application No. 39803/80), the power of one of the two lens components is weak and, consequently, the amount of movement of the other lens compoennt becomes large.
Another focusing method is known for a telephoto lens system comprising a front lens group having positive refractive power, an intermediate lens group having negative refractive power and a rear lens group having positive refractive power, i.e., a method to focus the lens system by moving the negative lens group (the intermediate lens group) toward the image side. However, when the negative lens group is moved toward the image side, heights of paraxial rays which enter the negative lens group become low and the diverging effect becomes weak. As a result, spherical aberration tends to be undercorrected and this tendency becomes stronger when the lens system is focused on an object at a short distance. To cancel the above-mentioned tendency that spherical aberration is undercorrected, it is so arranged that the negative intermediate lens group is composed of a plural number of lens components, a surface having strong converging effect is provided therein, and said negative lens group is moved toward the image side inclusive of said surface so as to thereby reduce the converging effect of said surface and to thereby cancel the undercorrecting tendency of spherical aberration. In this method, however, the lens system is focused by substantially moving the negative lens component only, and the amount of movement of the negative lens component becomes large. Therefore, the rigidity of lens tube becomes low when a cam is formed on it, and the rotary angle of focusing ring becomes large.